542 ' KNOWER. [Vol. XVI. 



ing in anamniote vertebrates, where the mechanical conditions, 

 as far as this theory goes, appear to be much the same as in 

 araniote vertebrates. 



Apparently similar conditions of pressure and mechanical 

 strains would be brought to bear on the embryonic areas of the 

 myriopods, the apterygota, or Crustacea, as are claimed to neces- 

 sarily force the formation of the amnion of insects, but no 

 amnion appears in the former groups. The invaginations 

 which do occur (to form the eyes, the digestive tract, etc.) in 

 some of the rapidly proliferating areas of the decapod blasto- 

 derm would be generally thought to necessitate something 

 more than such an enumeration of mechanical strains to 

 explain them. 



In those highly specialized insects that entirely lack an 

 amnion, its failure to appear is even more marked. Here, 

 within the same group, there are forms which, in the face of 

 the forces above stated as sufficient to produce an amnion, 

 have none. 



The effort to apply such a simple mechanical explanation 

 to the origin of various organic larval structures may seem 

 plausible at first sight ; but, carried to its logical limit, not 

 so much so. Why stop at the structures mentioned } Might 

 not the germ-layers, the central nervous-system, as well as 

 other such rapidly proliferating areas, be as readily included .-• 

 Heymons, as I have shown, has already attempted an affirma- 

 tive answer for the origin of the gastrula groove. 



a. 2. Turning from such general considerations to my own 

 special results, the formation of the caudal flexure of the Ter- 

 mite seems a case in point. 



This ventral flexure of the tail end of the embryo, as I have 

 pointed out, at first seems just as reasonably to be ascribed, 

 solely and directly, to a necessary result of pressure or me- 

 chanical strain as the instances referred to by Wheeler. A sin- 

 gle unusual specimen proved beyond doubt such a conclusion to 

 be false, and that what might appear superficially to be a neces- 

 sary method of growth could be accomplished in an entirely dif- 

 ferent manner. It was certainly proved to be independent of the 

 resistance of the chorion, which seemed so determinative at 



